The objective of these investigations is to evaluate certain immunologic aspects of gram-negative bacilli and their relation to the frequency and outcome of bacteremia with these organisms. Relatively nonspecific defense mechanisms rather than specific antibodies and antigens will be evaluated. The specific aims of the investigation include: 1. Characterization of antibody response against the causative organism of gram-negative bacteremia. 2. Evaluation of whether isohemagglutinins to blood group antigens influence the frequency or severity of bacteremia with gram-negative bacilli whose cell walls contain antigens cross-reacting with blood group substances. The effectiveness of immunization with blood group antigens against experimental bacilli possessing blood group antigens will also be evaluated. 3. Determination of antibody response to the "core portion" of endotoxin in patients with gram-negative bacteremia. The frequency and severity of gram-negative bacteremia will also be correlated with antibody levels to "core endotoxin". 4. Evaluation of antibody response to the "common enterobacterial antigen" in gram-negative bacteremia. Antibody titers to this "common antigen" will be correlated with both the outcome and frequency of gram-negative bacteremia. The effects of immunization with common antigen on experimental infections with bacteria possessing this antigen will be studied.